A diffusion barrier layer is required during copper metallization in the processing of integrated circuits and semiconductor devices to prevent Cu diffusion under electrical bias during device operation. When copper contacts silicon material, copper silicide is formed, which consumes Cu and deteriorates electrical conduction. Cu incorporation into dielectric layers degrades the dielectric properties of the insulating layer, causing leakage currents, and leading to inferior device performance and failure. With feature sizes of devices decreasing down to below 20 nm, the thickness of the diffusion barrier layers has been further reduced to less than 1-2 nm. To achieve such a thin blocking layer is challenging.
Previous approaches to providing diffusion barriers have included the use of metal alloys. For example, Ti or Ta compounds or Cu alloys have been utilized. However, the 10-30 nm thickness of the barrier layer required when using metal alloys is incompatible with use with sub-100 nm feature sizes in next-generation devices. In addition, obtaining such a thin barrier by conventional vapor phase deposition methods is difficult without compromising the barrier layer structure or conformality and in preparing high aspect ratio features.
U.S. Pat. No. 7,202,159 to Ganapathiraman describes the use of self-assembled monolayers (SAMs) formed from substituted silanes as diffusion barriers. The silanes include organic moieties comprising alkyl or aromatic groups to provide a diffusion barrier function, and react with the silicon oxide substrate by reaction of an O-alkyl leaving group. However, only the aromatic groups tethered to the substrate via an alkyl linker showed any efficacy at preventing metal diffusion. Krishnamoorthy, et al. reported that MOS capacitors with SAMs with short tail lengths or aliphatic terminal groups were ineffective in hindering Cu diffusion, while SAMs terminated by aromatic rings showed longer failure times. The authors concluded that steric hindrance due to the terminal groups in the SAMs is responsible for the barrier properties. (Krishnamoorthy, A., et al. 2001 Appl. Phys. Lett. 78, 2467).
Khaderbad et al. describe metallated porphyrins as self-assembled monolayers on silicon substrates, and their use as Cu diffusion barriers. (Khaderbad, M. A et al. “Metallated Porphyrin Self Assembled Monolayers as Cu Diffusion Barriers for the Nano-scale CMOS Technologies”, Proceedings of the 8th IEEE Conference on Nanotechnology, Aug. 18-21, 2008, Arlington, Tex. USA.) This reference teaches that the presence of aromatic rings sterically hinders Cu diffusion between molecules through the SAM layer. In addition, the authors suggest that the steric effect may be supplemented by the presence of metal ions in the center of the porphyrin macrocycle, and by the role of Cu—N bonds formed in the pyrrole subunits.
Caro et al. describe the use of a 3-aminopropyltrimethoxysilane-derived. SAM as a barrier against copper diffusion. The authors state that the SAM forms a carbon rich interface with the Cu overlayer, and the Cu did not penetrate through the SAM to the underlying silicon. These authors also describe use of a 3-mercaptopropyltrimethoxysilane-derived SAM in preparation of a Cu diffusion barrier, but report that this SAM did not prevent Cu penetration and silicide formation, and hence was ineffective. (Caro, A. M. et al. 2008 Microelectronic Engineering 85, 2047). Cam et al. also describe use of 3-mercaptopropyltrimethoxysilane-derived SAM as a sacrificial layer in preparation of a dual damascene integration scheme. (Caro, A. M., et al, 2010 Adv. Funct. Mater. 20, 1125).